1. Field of the Invention
This invention generally relates to wafer inspection using free-form care areas.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers. One important goal for any wafer inspection system is to suppress nuisance defects. Nuisance defects are those detected events which may not be relevant to semiconductor yields. Nuisance defects may be caused by wafer noise and system noise or are physical objects on the wafer. Nuisance defects may appear anywhere on a wafer. Some defects of interest (DOI) may appear only at certain locations on a wafer. Context information for a DOI may be used as prior knowledge for defect detection. Several approaches that use context information have been developed to detect defects. One such approach uses graphical data stream (GDS) data or design information to find hot spots where defects may occur at a higher probability and to inspect for defects around the hot spots.
There are, however, a number of disadvantages to such approaches. For example, for approaches that work with GDS data, GDS information may not be available in all circumstances such as for defect engineers in semiconductor fabrication plants. In addition, in order to detect defects with weak signal using the current approaches, huge amounts of nuisance defects may be detected. The defect signal may be defined as the maximum gray-level difference at defective pixels between an image with a defect and a reference image without the defect. The reference image is spatially-aligned with the defect image and may be acquired from neighboring dies or from multiple dies on the wafer. In all previous defect detection approaches, care areas are defined in terms of rectangles. Care areas are masks under which image pixels are inspected to detect defects. In some situations, DOIs are located along wafer structures which are not rectangular. If a care area with a rectangular shape is used to cover the DOI areas, extra pixels which may be nuisance sources will be inspected.
Accordingly, it would be advantageous to develop methods and/or systems for detecting defects on wafers that do not have one or more of the disadvantages described above.